1. Field of the Invention
The present invention relates to a semiconductor device provided with a function for controlling by a transistor a current supplied to a load. In particular, the invention relates to a pixel formed of a current driving light emitting element of which luminance changes depending on current, and a semiconductor device including a signal line driver circuit.
2. Description of the Related Art
In recent years, a self-luminous type display device of which pixel is formed of a light emitting element such as a light emitting diode (LED) is attracting attention. As a light emitting element used in such a self-luminous type display device, an organic light emitting diode (OLED), an organic EL element, and an electroluminescence (EL) element are attracting attention and becoming to be used in an organic EL display and the like.
Being self-luminous type, such a light emitting element as an OLED has a pixel higher in visibility and faster in response without a need of a backlight as compared to a liquid crystal display. Further, the luminance of a light emitting element is controlled by a current value flowing through it.
In a display device using such a self-luminous type light emitting element, a passive matrix method and an active matrix method are known as its driving method. The former has a simple structure, but has a problem such that the realization of a large and high definition display is difficult. Therefore, the active matrix method is actively developed in recent years in which a current flowing to the light emitting element is controlled by a thin film transistor (TFT) provided in a pixel circuit.
In the case of a display device of the active matrix method, there is a problem that a current flowing to a light emitting element varies due to variations in current characteristics of driving TFTs, thereby a luminance varies.
That is, in the case of such a display device of the active matrix method, a driving TFT which drives a current flowing to the light emitting element is used in a pixel circuit. When characteristics of these driving TFTs vary, a current flowing to the light emitting element varies, which varies a luminance. Therefore, various circuits have been suggested in which a current flowing to a light emitting element does not vary, thereby, variations in luminance can be suppressed even when characteristics of driving TFTs in pixel circuits vary, which can suppress variations in luminance.
[Patent Document 1]
Published Japanese Translation of PCT International Publication for Patent Application No. 2002-517806
[Patent Document 2]
International Publication WO01/06484
[Patent Document 3]
Published Japanese Translation of PCT International Publication for Patent Application No. 2002-514320
[Patent Document 4]
International Publication WO02/39420
Patent Documents 1 to 4 each discloses a structure of an active matrix type display device. Patent Documents 1 to 3 disclose a circuit configuration in which a current flowing to a light emitting element does not vary due to variations in characteristics of driving TFTs provided in pixel circuits. This configuration is referred to as a current write type pixel or a current input type pixel. Patent Document 4 discloses a circuit configuration for suppressing variations of a signal current due to variations of TFTs in a source driver circuit.
FIG. 169 shows a first configuration example of a conventional active matrix type display device disclosed in Patent Document 1. The pixel shown in FIG. 169 comprises a source signal line 16901, first to third gate signal lines 16902 to 16904, a current supply line 16905, TFTs 16906 to 16909, a capacitor 16910, an EL element 16911, and a current source 16912 for inputting a signal current.
A gate electrode of the TFT 16906 is connected to the first gate signal line 16902, a first electrode thereof is connected to the source signal line 16901, and a second electrode thereof is connected to a first electrode of the TFT 16907, a first electrode of the TFT 16908, and a first electrode of the TFT 16909. A gate electrode of the TFT 16907 is connected to the second gate signal line 16903 and a second electrode thereof is connected to a gate electrode of the TFT 16908. A second electrode of the TFT 16908 is connected to the current supply line 16905. A gate electrode of the TFT 16909 is connected to the third gate signal line 16904 and a second electrode thereof is connected to an anode of the EL element 16911. The capacitor 16910 is connected between the gate electrode and an input electrode of the TFT 16908 and holds a gate-source voltage of the TFT 16908. The current supply line 16905 and a cathode of the EL element 16911 are inputted with predetermined potentials respectively and have a potential difference from each other.
An operation from writing a signal current to light emission is described with reference to FIG. 172. Reference numerals denoting each portion in the drawing correspond to those in FIG. 169. FIGS. 172A to 172C each schematically shows a current flow. FIG. 172D shows a relationship of a current flowing each path when writing a signal current. FIG. 172E shows a voltage accumulated in the capacitor 16910 when writing a signal current, which is a gate-source voltage of the TFT 16908.
First, a pulse is inputted to the first gate signal line 16902 and the second gate signal line 16903 and the TFTs 16906 and 16907 are turned on. At this time, a current flowing through the source signal line, that is a signal current is denoted as Idata.
As the current Idata flows the source signal line, the current path is divided into I1 and I2 as shown in FIG. 172A. These relationships are shown in FIG. 172D. It is needless to say that Idata=I1+I2 is satisfied.
A charge is not held in the capacitor 16910 at the moment the TFT 16906 is turned on, therefore, the TFT 16908 is off. Accordingly, I2=0 and Idata=I1 are satisfied. In the meantime, current only flows into the capacitor 16910 to be accumulated therein.
After that, as the charge is gradually accumulated in the capacitor 16910, a potential difference starts to generate between both electrodes (FIG. 172E). When the potential difference between the both electrodes reaches Vth (a point A in FIG. 172E), the TFT 16908 is turned on and I2 generates. As described above, as Idata=I1+I2 is satisfied, current still flows and a charge is accumulated in the capacitor while I1 decreases gradually.
The charge keeps being accumulated in the capacitor 16910 until the potential difference between the both electrodes thereof, that is a gate-source voltage of the TFT 16908 reaches a desired voltage, that is a voltage (VGS) which can make the TFT 16908 supply the current Idata. When the charge stops being accumulated (a point B in FIG. 172E), the current I1 stops flowing and a current corresponding to VGS at that time flows through the TFT 16908 and Idata=I2 is satisfied (FIG. 172B). Thus, a steady state is achieved. At last, selections of the first gate signal line 16902 and the second gate signal line 16903 are terminated to turn off the TFTs 16906 and 16907.
Subsequently, a light emitting operation starts. A pulse is inputted to the third gate signal line 16904 to turn on the TFT 16909. As the capacitor 16910 holds VGS which is written before, the TFT 16908 is on and the current Idata flows from the current supply line 16905. Thus, the EL element 16911 emits light. Provided that the TFT 16908 is set to operate in a saturation region, Idata keeps flowing without changing even when a source-drain voltage of the TFT 16908 changes.
In this manner, an operation to output a set current is hereinafter referred to as an output operation. A merit of the current write type pixel of which example is shown above is that a desired current can be accurately supplied to an EL element because a gate-source voltage required to supply the current Idata is held in the capacitor 16910 even when the TFT 16908 has variations in characteristics and the like. Therefore, luminance variations due to the variations in characteristics of TFTs can be suppressed.
The aforementioned examples relate to a technology for correcting a change of current due to variations of driving TFTs in pixel circuits, however, the same problem occurs in a source driver circuit as well. Patent Document 4 discloses a circuit configuration for preventing a change of a signal current due to variations of the TFTs in the source driver circuit generated in fabrication.